First select from tab2, those which are not in tab1, then just join tab1 to that list. The # is a place-holder and the & designates it as a pure function: go through the list tab2, and insert each element in that list into the place-holder inside MemberQ and compare it to all the members of tab1. Finish evaluating the expression.

Not sure what you want. Try this. Can you interpret the functional programming: sequence = ({#1} & ) /@ Take[new, -7]?
It's very cryptic I know. Only way is practice: Establish the pure function {{#}&. Now apply (/@ is shorthand for Apply) this function to each element in the list formed by taking the last seven elements from new. That gives {{2},{3},{4},{6},{7},{8},{10}}. The format needed for Extract.